Note: Descriptions are shown in the official language in which they were submitted.
WO 94!08235 PCT/US93/08836
~1~~11:~
G
r~acxg~ounn of the Invention
This invention relates to analyzers for
determining the level of a selected chemical ~n a process
.fluid. ~ -
A typical analyzer for measuring the level of a
chemical in a process fluid includes a voltammetric
sensor that supports one or more electrodes in
5.0 electrolytic contact with the fluid. The electrodes~are
immersed in an electrolytic solution of fixed composition
at one end of the sensor and are separated from the
process fluid by a membrane that is permeable to the
selected chemical. This type of sensor is known as a
Clark cell when the chemical to be measured is oxygen or
its allotropes such as ozone.
When the sensor is inserted in the process fluid
and an oxygen-permeable membrane is used, oxygen in the
fluid diffuses through the membrane into the electrolytic
solution. As a result, when the electrodes are energized
(with either D.C. or pulsed potential) they produce an
electrical signal that is proportional to the level of
oxygen in the electrolyte, and hence the amount of oxygen
in the process fluid.
Such sensors may be adversely affected by membrane
breakage or fouling. For example, the membrane may
become fouled during.use by materials (such as dirt, oil,
grease, sludge, etc.) in the process fluid that collect
on the membrane, reducing its permeability. As a result,
the signal produced by the sensor will no longer
accurately reflect the oxygen level in the process fluid,
thereby leading to measurement errors. This problem is
particularly acute when the process fluid comprises
sludge-laden waste water in sewage treatment plants.
CA 02124113 2003-07-09
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Summarv of the Invention
Various embodiments of this invention provide an
apparatus for detecting a selected chemical in a fluid,
comprising a plurality of electrodes separated from said
fluid by a membrane that is permeable to said selected
chemical, each of said electrodes when energized producing a
signal in response to said selected chemical in the fluid, a
power source for energizing said electrodes, and means for
determining a level of said selected chemical in said fluid
based on said signal produced by a first one of said
electrodes and detecting whether said membrane is faulty
based on said determined level and said signal produced by a
second one of said electrodes.
Various embodiments of this invention provide an
apparatus for detecting a selected chemical in a fluid,
comprising a housing that is adapted to be inserted in said
fluid, said housing supporting a plurality of electrodes
separated from said fluid by a membrane that is permeable to
said selected chemical, a first one of said electrodes being
adapted to be energized for first duration of time
sufficient to enable said first electrode to produce a first
signal that indicates a level of said selected chemical in
said fluid, a second one of said electrodes being adapted to
be energized for a second duration of time sufficient to
enable said second electrode to produce a second signal that
when compared with said first signal provides an indication
of whether said membrane is faulty.
Various embodiments of this invention provide a method
for detecting a selected chemical in a fluid, comprising
providing a plurality of electrodes separated from said
fluid by a membrane that is permeable to said selected
chemical, each of said electrodes when energized producing a
signal in response to said selected chemical in the fluid,
CA 02124113 2003-07-09
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energizing said electrodes, determining a level of said
selected chemical in said fluid based on said signal
produced by a first one of said electrodes, and detecting
whether said membrane is faulty based on said determined
level and said signal produced by a second one of said
electrodes.
Various embodiments of this invention provide a method
for detecting a selected chemical in a fluid, comprising
providing a plurality of electrodes separated from said
fluid by a membrane that is permeable to said selected
chemical, each of said electrodes when energized producing a
signal in response to said selected chemical in the fluid,
energizing a first one of said electrodes for a time
sufficient to enable said signal produced thereby in
response to said selected chemical to reach a steady-state
value and determining a level of said selected chemical
based on said signal produced by said first electrode,
energizing a second one of said electrodes for a time
insufficient to enable said signal produced thereby in
response to said selected chemical to reach a steady- state
value, and detecting whether said membrane is faulty based
on comparing said signal produced by said second electrode
with said determined level.
CA 02124113 2003-07-09
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This invention provides an efficient and highly
accurate technique for determining whether the sensor
membrane has become faulty by providing the sensor with
multiple electrodes -- one of which is used to measure
the level of a selected chemical in the process fluid,
and another electrode serving as a diagnostic device that
enables faults in the membrane to be detected.
In one general aspect of this concept, the signal
produced by a first electrode when energized is used to
determine the selected chemical level of the process
fluid, and the. signal produced by a second electrode when
energized is used in conjunction with the determined
level to detect whether the membrane is faulty.
Preferred embodiments include the following
features.
The signal produced by the second electrode is
analyzed to derive an expected level of the selected
chemical (which is, e.g., oxygen or an allotrope thereof,
such as ozone). The membrane is detected to be faulty if
the oxygen level as determined from the signal produced
by the first electrode differs from the expected level by
more than a selected amount. Preferably, a portion of
the signal produced by the second electrode is integrated
to produce the expected level. The selected amount is
based on a calibrated ratio between the determined level
and the expected level and is between 10% and 50% of the
calibrated ratio. An alarm is generated if the membrane
is determined to be faulty.
The first and second electrodes are energized for
mutually exclusive periods of time and for different
durations. The first electrode is energized for longer
durations than the second electrode. The first electrode
is energized for a time sufficient to allow the signal
that it produces to reach a steady-state value, while the
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~V~ 9d/08235 PCT/US93/08836
212~~ ~ 3
amount of time that the second electrode is energized is
insufficient to allow its signal to reach a steady-state
value. For example, the first electrode is energized for
intervals of 15 minutes or more, and the second electrode
is energized on the order of 10 seconds between
successive energizations of the first electrode. The
user can select these durations. The signal produced by
the second electrode is analyzed during only a portion of
the time that the second electrode is energized (e. g.,
between two and four seconds after the second electrode
is energized).
During operation of the sensor, the membrane may
become coated by materials in the fluid sample, and the
membrane is subject to fouling if such material reduces
the permeability of the membrane to oxygen. The membrane
is designated a~ being fouled -- and thus faulty -- if
the determined level and the signal produced by the
second electrode differ by more than the selected amount.
The temperature of the fluid is also determined
(e.g., by a thermistor, which measures the temperature of
the electrolyte solution in which the electrodes are
disposed). At least the signal produced by the first
electrode is corrected according to temperature to
provide the user with a temperature-compensated
measurement of the oxygen level. The oxygen level is
presented in any convenient form (e.g., as a
concentration in parts-per-million (ppm) or as a percent
saturation of the fluid with oxygen).
The first and second~electrodes are spaced by an
amount selected to allow each electrode to measure the
amount of oxygen in different regions of the electrolyte
without interference from the other electrode.
Preferably, the electrodes are each annular and are
disposed coaxially with respect to each other.
WO 94/08235
PCT/ US93/0883~~;
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One embodiment of the invention features detecting
breakage of the membrane. Because the process fluid
typically has a different electrical resistance than the
electrolyte within which the first and second electrodes
are immersed, membrane breakage is detected by,
determining the electrical resistance between~the
electrodes and comparing the resistance to an expected
(i.e., reference) value. If a discrepancy is found, the
membrane is determined to be broken. The resistance
measurement is made during the 10 second period that the
second electrode is energized, but after the 2-4 second
time window in which the signal from the second electrode
is analyzed to diagnose membrane fouling.
The sensor also includes a third electrode (known
as an auxiliary electrode) disposed within the
electrolyte with the first and second electrodes for
supplying electrical current to energize the first and
second electrodes. A fourth, reference electrode is
disposed adjacent to first and second electrodes, and the
electrical current applied to the third electrode is
controlled based on the electrical potential developed
between the reference electrode and the energized first
and second electrodes.
The electrical current for the third electrode is
generated .by a driver. ?boss of the electrolyte solution
increases the load on the driver, and thus the output
voltage of the driver is monitored and compared with a
threshold. If the threshold is exceeded, loss of the
electrolyte is deemed to have occurred, and an alarm is
activated to so notify the user.
The sensor includes a housing for supporting the
first and second electrodes and the electrolyte solution
in a cavity thereof, a portion of which is bounded by the
membrane. A second membrane (called a diaphragm) bounds
another portion of the cavity and is maintained at a
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tension less that a tension of the first membrane. The
slack diaphragm expands preferentially to the membrane in
response to temperature-induced expansion of the
electrolyte (and, to a lesser degree, the presence of
bubbles in the electrolyte solution), thereby maintaining
a fixed spacing between the oxygen permeable membrane and
the electrodes.
The invention provides an accurate, real time
technique for determining whether the membrane has become
fouled without interfering with the oxygen level ' '
measurement in any significant way. Because the second
electrode is energized for only brief periods between
relatively long measurement cycles in which the first
electrode is energized, the membrane fouling diagnosis is
essentially transparent to steady-state measurement of
the oxygen level provided by the first electrode. The
user can select the tolerable degree of membrane fouling
based on the environment in which the sensor is used.
Other features and advantages of the invention
will become apparent from the following detailed
description, and from the claims.
Brief Description of the Drawings
Fig. 1 is a functional block diagram of a chemical
analyzer according to the invention, which includes a
sensor for measuring the level of oxygen in a process
fluid.
Fig. 2 is a partial cross-sectional view of a
portion of the sensor of the chemical analyzer of Fig. 1.
Fig. 3 is an end view of the sensor of Fig. 2 that
shows the arrangement of two of the electrodes of the
sensor.
Fig. 4 illustrates the current-time response of
one of the electrodes of the sensor of Figs. 1 and 2.
Fig. 5 is a flow chart usoful in understanding how
the analyzer of Fig. 1. diagnoses membrane fouling.
WO 84/08235 PCT/ 1JS93/0883b- --.
_ 6 _
Fig. 6 illustrates the current-time response of
another electrode of the sensor of Figs. 1 and 2.
Fig. 7 shows a calibration chart useful in '
understanding how membrane fouling is diagnosed.
Fig. 8 is a functional block diagram Qf circuitry
for detecting a rupture in the membrane of tie sensor of
Figs. 1 and 2.
Fig. 9 is a functional block diagram of circuitry
for detecting the presence of a gaseous bubble in the
sensor of Figs. 1 and 2.
Description of the Preferred Embodiments
Referring to Fig. 1, an analyzer 10 for measuring
the level of a selected chemical in a process fluid (not
shown) includes a sensor 11 that contains a measurement
electrode 12 and an adjacently disposed test electrode 14
immersed within an electrolyte solution 16 (only a
- portion of which is depicted) and enclosed by a membrane
18. Membrane 18 is permeable to the selected chemical,
which is, for example, oxygen or its allotropes. Also
enclosed by membrane 18 are an auxiliary electrode 20 for
supplying energizing current to electrodes 12, 14, and a
reference electrode 22 for allowing the voltage at
electrodes 12, 14 to be maintained relatively constant
for purposes to be described. Each electrode 12, 14 when
energized produces an electrical current the amplitude of
which is proportional to the level of oxygen that has
diffused into the electrolyte solution from the process
f luid.
Electrodes 12, 14 are selectively energized by
measurement/diagnosis controller 24 at mutually exclusive
times and for different durations. Controller 24
performs this function by selectively coupling electrodes
12, 14 to a current-to-voltage converter 26 (switch 28
schematically represents this function), which completes
the electrical circuit with auxiliary electrode 20 and
-''O 94/08235 ~ PCT/LS93/08836
transforms the current produced by the selected electrode
to a voltage that represents the level of oxygen in the
process fluid. A variable resistor 27 in converter 26
allows the user to set the gain of the current to voltage
conversion. ,
The voltage is digitized by an analog-to-digital
(A/D) converter 30 and analyzed by a processor 32, which
also controls the operation of controller 24 and A/D
converter 30. Processor 32 operates under the control of
a program 31 stored in memory 33. It will be appreciated'
that the functions of at least controller 24 and
processor 32 can be implemented by a microprocessor;
separate units are shown for ease of explanation.
The operation of analyzer 10 is discussed in
detail below. Suffice it here to say that measurement
electrode 12 is energized in a manner selected so that
electrode 12 produces a signal representative of the
oxygen level in the process fluid, and test electrode 14
is periodically energized in place of measurement
electrode 12 to develop a signal used by processor 32 in
conjunction with the previously obtained signal from
measurement electrode 12 to determine whether membrane 18
has become faulty (e. g. fouled or coated with material
such as dirt, sludge, waste, oil, grease, etc.).
More specifically, measurement electrode 12 is
energized for a time period sufficient to allow the
current response thereof to reach a steady-state level.
For example, electrode 12 is energized for between 15
minutes and 1 hour or more. As a result, in the absence
of a fault in membrane 18 the current produced by
electrode 12 (and hence the voltage applied to processor
32) is linearly related to the rate at which oxygen
diffuses from the process fluid to electrolyte 16 across
membrane 18, and thus the current is proportional to the
oxygen level in the fluid. Processor 32 displays the
WO 9~/~8~~~'~'~ PCT/US93/0883~~w~
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measured oxygen level to the user (e.g., as a
concentration in parts per million (ppm) or as a percent
saturation) by illuminating a display 34 on user
interface 35 (which is located, e.g., on the front panel
of a housing that contains the circuitry discussed
above). Alternatively, the measured oxygen~level may be
sent to a process control computer for adjusting the
oxygen level in the process fluid, to a data logger, or
to a printer (none of these devices are shown).
If membrane 18 becomes faulty (for example, if
membrane 18 becomes fouled with a coating of material
that interferes with oxygen permeability), the rate of
oxygen diffusion across membrane 18 slows, thereby
decreasing the steady-state signal produced by
measurement electrode 12 so that it no longer accurately
represents the oxygen level of the process fluid. To
enable analyzer l0 to diagnose membrane fouling,
processor 32 periodically commands controller 24 to
briefly energize test electrode 14 (by inserting test
electrode 14 into the circuit of auxiliary electrode 20
in place of measurement electrode 12). Test electrode 14
is energized sufficiently briefly (e.g., on the order of
10 seconds) so that its current response does not reach a
steady-state level. As a result, the oxygen level
measured by test electrode 14 is the equilibrium level of
the oxygen established in electrolyte 16 during the time
. that measurement electrode 12 was energized, rather than
the rate of oxygen diffusion across membrane 18. Thus, .
any fouling of membrane 18 does not appreciably degrade
the current level produced by electrode 14. Accordingly,
processor 32 determines whether membrane 18 has become
fouled by comparing the signal produced by test electrode
-- 14 with the oxygen level as measured with electrode 12 in ,
a manner described in detail below.
n'''i3 94/08235 ~ ~ ~ ~ PCT/US93/08836
_ g _
If the signal produced by test electrode 14
indicates that the process fluid contains an appreciably
greater level of oxygen than that indicated by the
steady-state current level produced by measurement
electrode 12, processor 32 determines that membrane 18
has become fouled and alerts the user by generating an
alarm 36. Alarm 36 may be visual (for example, processor
32 may alter the oxygen level display 34 from a
continuous to a blinking signal). In addition (or
alternatively), alarm 36 may be audible, such as a tone '
38. The user can respond to the alarm by either cleaning
membrane 18 or replacing it with a new membrane.
Referring to Figs. 2 and 3, the end of sensor 11
that is constructed to be inserted into process fluid 40
is shown in detail. Sensor 11 includes a hollow housing
42 made from Kynar~, a plastic PVDF (poly ~~inylidene
difluoride) commercially available from the Penwalt
Corporation. The distal end of housing 42 supports a
pair of coaxial gold or platinum rings that comprise
measurement electrode 12 and test electrode 14,
respectively. Measurement electrode 12, is disposed
radially inwardly of test electrode 14, but the positions
of electrodes 12, 14 may of course be reversed. An
interior chamber 44 of housing is filled with electrolyte
solution 16 which is, for example, a 2 molar KC1
(potassium chloride) that supports the electrochemical
reaction of oxygen at electrodes 12, 14. Solution 16 may
also include a surfactant and an algicide.
Membrane 18 is stretched tightly over the face 48
of housing 42 in which electrodes 12, 14 are embedded and
is secured in place by a collar 50 that is attached to
housing 42 with threads (not shown). Membrane 18 is thin
(e. g., between 0.5 mils and 5 mils thick) and is made
from any suitable material that is highly permeable to
oxygen. For example, membrane 18 is Teflon~, silicone
1~V0 94/08235 PCT/US93/0883~ ~:.,
'_
-
rubber, or polyethylene. The spacing between membrane 18
and electrodes 12, 14, is quite narrow (and is shown
greatly exaggerated for purposes of illustration) but is
sufficient to allow a thin film of electrolyte solution
5 16 to form therebetween by capillary action.,_
A spacing 8 (e. g., 1/8 inch) separates measurement
electrode 12 and test electrode 14 so that electrodes 12,
14 are exposed to different annular regions 45, 46 of a
the film of electrolyte solution 16. When measurement
10 electrode 12 is energized, oxygen that permeates membrane
18 into the film of electrolyte 16 in region 45 is
electrochemically consumed as the oxygen reaches
measurement electrode 12. Because test electrode 14 is
de-energized during this time, oxygen that diffuses
through membrane 18 into annular region 46 of electrolyte
16 is not consumed. Thus, the oxygen in region 46 is in
equilibrium with the concentration of oxygen in process
fluid 40. It is this level of oxygen that is measured
when test electrode 14 is briefly energized to diagnose
membrane fouling. The seal between membrane 18 and
housing 42 and the energization of measurement electrode
12 serve to avoid lateral diffusion of oxygen to test
electrode 14 from fluid 40 and the electrolyte in cavity
44, respectively.
The walls of housing 42 are hollow to accommodate
wires 47, 49 that provide the electrical connections
between switch 28 (Fig. 1) electrodes 12, 14,
respectively. In addition, a thermistor 51 is suspended .
near the distal end of housing 42 by a pair of wires 53.
Processor 32 (Fig. 1) uses the temperature measurement of
electrolyte 16 provided by thermistor 51 (which indicates
the temperature of fluid 40) to apply temperature
correction to the oxygen level measurement, as described ,
below.
'-'j0 94/0823; 212 4 ~ ~ ~ PCT/US93/08836
- 11 -
A tube 52 is suspended within chamber 44 and
supports reference electrode 22 (Fig. 1), which is a
silver wire coated with silver chloride immersed in an
electrolyte solution 56 of KC1. A porous ceramic wick 58
mounted in the distal end of tube 52 maintains
'electrolytic contact between solutions 16 and 56, and
limits the internal diffusion of KC1 between chamber 44
and the interior of tube 52. As explained below, this
allows reference electrode 22 to be used to control the
potential at electrodes 12, 14 when electrodes 12, 14 are'
energized.
Auxiliary electrode 20 (Fig. 1) is a silver wire
in the form of a coil that is wrapped around tube 52
(only a portion of each winding is shown for clarity).
The end 62 of the silver wire that forms auxiliary
elecfirode 20 is disposed adjacent to the distal end of
tube 52. Silver is used to avoid adding oxygen to
electrolyte 16 (which would occur if material such as
platinum or gold were to be used for electrode 20 and
would disrupt the measurement) when auxiliary electrode
20 oxidizes during operation.
A pressure relief diaphragm 64 is held over a side
opening 66 in chamber 44 by a plug 68. Diaphragm 64 is
held at a much lower tension than membrane 18 so that
diaphragm 64'expands and contracts preferentially to
membrane 18 in response to changes in the pressure of
electrolyte 16. For example, a gaseous bubble (not
shown) trapped in chamber 44 will cause diaphragm 64
rather than membrane 18 to~expand in response to
' 30 temperature changes or pressure (depth of immersion)
variations experienced by sensor 11. This helps maintain
a constant spacing between membrane 18 and electrodes 12,
14 despite pressure changes in electrolyte 16. Because
the rate of oxygen diffusion of from process fluid 40 to
electrodes 12, 14 is a function of the spacing of
W~ 94/0823 PCT/US93/0883&~~~.
212 ~~.~.3
- - 12 -
membrane 18, a nonvariable spacing is critical to the
accuracy of the oxygen level measurement.
Referring also to Fig. 1, auxiliary electrode 20
and reference electrode 22 are connected to a power
source 70 that controls the level of current ,applied to
the energized electrodes 12, 14. Energizing current for
electrodes 12, 14 is provided by a D.C. voltage source 72
via a current driver 74. The polarity of the voltage
applied by source 72 is selected to polarize electrodes
12, 14 in the cathodic direction. The current response
of electrodes 12, 14 is linear with respect to the oxygen
level in process fluid 40 when electrodes 12, 14 are
energized at a potential of between about -0.6 volts and
-1.5 volts with respect to reference electrode 22.
Reference electrode 22 provides (through a high impedance
voltage follower 76) negative feedback for driver 74 to
maintain each electrode 12, 14 squarely within its linear
range when energized (e. g., at a potential of -0.7 volts
with respect to reference electrode 22).
Referring to Fig. 4, in operation, processor 32
begins the measurement process at time t=0 by commanding
controller 24 to connect measurement electrode 12 to
current to voltage converter 26 through switch 28,
thereby completing the circuit with auxiliary electrode
20 and energizing measurement electrode 12. The oxygen
concentration at the surface of electrode 12 is
immediately driven to zero as the 02 at the surface of
electrode 12 is faradaically reduced to OH- according to
the following equation:
OZ + 2H20 + 4e- ---> 40H-
As a result, iunnediately after time 0, measurement
electrode 12 produces a high level of current that decays
to a steady-state level according to curve 80. As shown
by region 82 of curve 80, the current is initially
capacitive (due to the charging of the double-charge
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layer at the surface of measurement electrode 12) as well
as faradaic (due to the reduction of oxygen present at
the surface of electrode 12). The immediate exhaustion
of oxygen at the surface of electrode 12 causes oxygen to
begin diffusing from process fluid 40 to electrolyte 16
across membrane 18, thereby slowing the rate~of current
decay. The current level continues to fall (as shown in
region 84 of curve 80) and ultimately reaches a steady-
state value that is proportional to the oxygen
ZO concentration gradient established across membrane 18 in
region 45 of electrolyte 16.
The current reaches its steady-state value
approximately 20 seconds after electrode 12 is energized.
At this time, the rate at which electrode 12 reduces
oxygen to OH- becomes fixed by the rate of oxygen
diffusion across membrane 18 and is linearly proportional
to the amount of oxygen in process fluid 40. Thereafter,
in the absence of a fault (such as fouling of membrane
18) or a change in the oxygen level of fluid 40, the
current level produced by electrode 12 will remain
constant, as shown by region 86 of curve 80.
Processor 32 measures the steady-state current
produced by electrode 12. As a result, processor 32
waits for about 1 minute after time 0 before beginning to
analyze the signals from electrode 12. The analog
current produced by measurement electrode 12 is converted
to a voltage by current to voltage converter 26, and this
voltage is repeatedly sampled and digitized by A/D
converter 30. Processor 32 controls A/D converter 30 to
sample the analog voltage every 33 milliseconds, but of
course other sampling periods may be employed.
Processor 32 converts the values (i.e., the
-- amplitudes) of the digital samples to derive the oxygen
concentration (in parts per million, ppm) of fluid 40 by
applying a constant of proportionality to the values.
WO 94/0823 PCT/1JS93/08836~ ,
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14
(The relationship between current and oxygen
concentration is described in Measurement of Dissolved
Oxyg~en, by Michael L. Hitchman, John Wiley & Sons, 1978.)
Because the temperature of sample 40 affects the current
produced by electrode 12, processor applies ~ temperature
correction factor (stored in a table 88 in memory 33) to
the derived oxygen concentration based on the temperature
measured by thermistor 51. (The circuitry for digitizing
the thermistor signal is not shown.) The temperature-
compensated oxygen level can then be displayed 34 as-a
concentration of oxygen in ppm, or alternatively
processor 32 may convert the oxygen level to another unit
of measurement (such as percent saturation) and display
the same. Periodically, or upon user command from an
input 37 (such as a keypad on the front panel), processor
32 sends the temperature measurement to display 34 in
place of the oxygen level.
Because the steady-state current 86 produced by
measurement electrode 12 is a function of the rate at
which oxygen diffuses across membrane 18, steady-state
current 86 will decrease if the diffusion of oxygen is
hampered. For example, oxygen-impermeable material
(e. g., dirt, algae, oil, etc.) in process fluid 40 that
collect on membrane 18 interfere with the diffusion of
oxygen, thereby "fouling" membrane 18 and decreasing the
current produced by electrode 12 so that the current no
longer accurately reflects the oxygen concentration in
fluid 40. This is shown by dashed portion 86~ of current -
curve 80 in Fig. 4. The problem of fouling is
' 30 particularly acute in applications in which sensor 10 is
used with highly contaminated process fluids 40 (such as
waste water in sewage treatment plants).
Referring also to Fig. 5, processor 32
periodically commands controller 24 to briefly energize
test electrode 14 via switch 28 in place of measurement
_ ~,s~~tl~
-"a0 94/08235 PCf/US93/08836
- 15 -
electrode 12 (step 100) and analyzes the resulting signal
produced by test electrode 14 to determine whether
membrane 18 has become fouled. The user can adjust the
intervals between successive energizations (e.g., 15
minutes to 1 hour or more) as well as the duration of
time that test electrode 14 is energized via input 37.
The duration that test electrode 14 is energized should
be insufficient for the current response of test
electrode 14 to reach a steady-state level, and should
also be sufficiently brief that measurement electrode 12
does not become discharged before it is re-energized by
controller 24 (thereby allowing measurement electrode 12
to rapidly return to its steady state response when it is
re-energized). Energizing test electrode 14 for 10
seconds between successive energizations of measurement
electrode 12 has been found to be satisfactory for these
purposes.
Referring also to Fig. 6, curve 90 illustrates the
current response of test electrode 14. Because of the
spacing between electrodes 12, 14, the oxygen
concentration gradient established in region 45 of
electrolyte by the steady-state operation of measurement
electrode 12 does not deplete the level of oxygen in
region 46 of electrolyte 16 disposed between test
electrode 14 and membrane 18. Thus, immediately prior to
the time that test electrode 14 is energized, the oxygen
level in electrolyte region 46 is at equilibrium with the
level of oxygen in fluid 40.
As explained above,. immediately after test
electrode 14 is energized, the oxygen at the surface of
test electrode 14 is depleted, causing electrode 14 to
produce a high level of current that decreases toward a
steady state level as oxygen diffuses across membrane 18.
For the first second or so after test electrode 14 is
energized (region 92 of curve 90), the current has a
..
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._
16 -
large capacitive component and is ignored by processor
32. After this interval and before the steady-state
current level is reached, the amount of current produced
by test electrode 12 is a function of the rate at which
the oxygen is exhausted in region 46 of electrolyte
between test electrode 14 and membrane 18, as shown by
region 93 of curve 90. Curve 90 represents approximately
a 50% oxygen Saturated sample. (For comparison, curve
90' shows the current response for a 100% oxygen
saturated sample 40, while a sample 40 that contains no
oxygen results in a current response shown by curve
90" . )
Processor 32 analyzes the current produced by test
electrode 14 during a portion of the time period that
electrode 14 is energized, for example, during time
window 94 of between 2 seconds and 4 seconds of the 10
second duration that electrode 14 is energized (step
102). During time window 94, A/D converter 30 provides
processor 32 with about 60 samples (step 104) (at the 33
millisecond sampling rate) of the voltage produced by
converter 26 in response to the current from test
electrode 14. Processor 32 applies temperature
correction (step 106) to the values of the samples (using
stored table 88)~and then integrates (step 108) the
samples by adding them together and dividing the sum by
the number of samples. The result corresponds to the
total charge (in Coulombs) 110 gathered by test electrode
14 during the 2-4 second time window.
Due to the relatively brief interval that test
electrode 14 is energized, the current produced by test
electrode 14 does not reach a steady-state level and thus
is not affected by the rate at which oxygen diffuses
through membrane 18 into electrolyte region 46. As a ,
result, applicant has found that total charge 110 is
substantially independent of the degree of fouling of
PCT/ US93/08836
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- 17 -
membrane 18. That is, for a given oxygen concentration
of process fluid 40, total charge 110 will be
substantially the same independently of whether membrane
18 is completely clean or is fouled. Thus, by comparing
total charge 110 with the previously obtained steady
__
state oxygen level measurement from measurement electrode
12 in the manner described below, processor 32 determines
whether membrane 18 has become fouled and, if so,
notifies the user by activating alarm 38.
Total charge 110 will rarely, if ever, equal-the
previously obtained steady state oxygen level measurement
and in fact should exceed it. Applicant has found that
the difference between total charge 110 and the steady
state oxygen level measurement increases as membrane 18
becomes fouled. The reason is that fouling causes the
steady-state current produced by measurement electrode 12
to decrease (as shown by curve 86~ in Fig. 5) but does
not cause a similar reduction in total charge 110, as
discussed above. Thus, processor 32 determines the ratio
between total charge 110 and the previously-obtained
steady-state measurement of the oxygen level (step 112),
and compares the actual ratio with the ratio that would
be expected for a non-fouled electrode (steps 114, 116).
If the actual ratio differs from the expected ratio by
more than a threshold amount, processor 32 determines
that membrane 18 has become fouled.
Referring to Fig. 7, processor 32 determines the
expected ratios between total charge 110 and the steady
state oxygen level measurements as a result of
calibration of analyzer 10 with a clean membrane 18.
During calibration, analyzer to makes a series of (e. g.,
four) steady-state measurements 130, 132, 134, 136 of the
- oxygen concentration of process fluid 4o using
measurement electrode 12 in the manner described above.
Each steady state measurement is made by energizing
WO g4/ ~ ~5~ ~ ~ ~ PCf/US9310883~-.
- 18 -
electrode 12 for 15 minutes. At the end of each 15
minute period, processor 32 stores the digitized,
temperature corrected current from measurement electrode
12 in a working area 98 of memory 33 (Fig. 1). Steady
state measurements 130-136 define a curve 14p that is
linear with respect to the oxygen concentration of
process fluid 40.
During calibration, analyzer 10 also performs a
series of (e. g., 4) total charge measurements 140, 142,
144, 146 in the manner discussed above using test
electrode 14. Total charge measurements 140-146 define a
linear curve 150 with respect to oxygen concentration and
are stored in working memory area 98. For any given
concentration of oxygen in fluid 40 (such as
concentration 152) the ratio between the total charge
value on curve 150 and the steady state value on curve
140 defines the expected ratio used by processor 32 in
step 116 (Fig. 5).
Thus, during operation, after processor 32
determines (step 112) the actual ratio between total
charge 110 and the previously measured oxygen
concentration, processor 32 determines (step 114) the
expected ratio for the previously measured oxygen
concentration using the calibration data stored in
working memory area 98. The linearity of curves 140, 150
with respect to oxygen concentration makes this a
relatively straightforward calculation. If the expected
ratio exceeds the actual ratio by a threshold of, e.g., .
30%, processor 32 determines that membrane 18 has become
fouled and activates the alarm (step 118). Otherwise,
processor commands controller 24 to re-energize electrode
12 (step 120) to begin the next measurement period.
_ The 30% threshold can be made stricter (e. g.,
reduced to 10% or less) or larger (e.g., increased to 50%
or more), depending upon the tolerance to fouling that
'"''s0 94/08235 2 ~ ~ 4 ~ ~ ~ PCf/U593/08836
- 19 -
the user desires. Requiring a closer match between the
expected and actual ratios will obviously alert the user
when only a small amount of fouling has occurred. This
may be advantageous in applications in which inaccuracies
caused by even minor fouling are not desirable. In
other, less stringent applications, the threshold may be
increased so that the user is alerted only when membrane
18 is severely fouled.
Other embodiments are within the scope of the
following claims. ' '
For example, analyzer 10 can be used to measure
the levels of chemicals other than oxygen by selecting
suitable electrolyte and electrode materials. Fouling
diagnoses can be based on the differences between the
expected values on curves 140, 150, rather than their
ratios.
Electrodes 12, 14 need not be energized for
mutually exclusive time intervals.
Processor 32 can alternatively use the fouling
determination to correct the oxygen level measurement.
Analyzer 10 can also perform other fault
diagnoses.
For example, referring to Fig. 8, circuitry 170
enables processor 32 to detect a rupture 172 in membrane
18 by detecting changes in the electrical resistance
between electrodes 12, 14. The resistance presented by
. electrolyte 16 (e.g., 2 molar KCl) is approximately 300
ohms. A rupture 172 in membrane 18 would permit some of
pracess fluid 40 (which is, e.g., waste water) to mix
with electrolyte 16, thereby altering the resistance
between electrodes 12, 14.
After each time window 94 (Fig. 6) but while test
electrode 14 is energized, processor 32 closes a switch
174 to apply an A.C. voltage from a source 176 across
electrodes 12, 14. The voltage produced by source 176 is
WO 94/08235 PCT/US93/088u
at a relatively high frequency (1000 Hz) and is small
(100mV) so as not to interfere with the current response
of either electrode. The resulting current between
electrodes 12, 14 is measured by sensor 178, and the
5 measurement is digitized (not shown) and applied to
'processor 32. Processor 32 applies temperature
correction to the current level according to the
temperature measured by thermistor 51. (Temperature
correction may be omitted, if desired.) If the
10 resistance indicated by the current differs from 300- ohms'
by more than a threshold amount (e.g., is outside of the
range of 100 ohms to 1000 ohms), processor 32 generates a
fault 180, which is displayed as an error message on
display 34 (Fig. 1).
15 Because the resistance measurement is made after
the current from test electrode 14 has been collected
during time window 94, it does not interfere with the
fouling diagnosis. Moreover, processor 32 opens switch
174 as soon as the resistance measurement has been
20 completed (which takes one second or less). Thus,
circuitry 170 is disconnected before measurement
electrode 12 is re-energized.
Referring to Fig. 9, circuitry 190 detects loss of
electrolytic solution 16 or the presence of one or more
gaseous bubbles 191 in electrolyte 16. Although, as
discussed above, pressure compensation diaphragm 64 (Fig.
2) helps maintain a fixed spacing between electrodes 12,
14 and membrane 18, the loss of electrolyte 16 or the
presence of bubbles 191 in electrolytic solution 16 has
other harmful effects. For example, loss of electrolyte
16 or the addition of gaseous bubbles increase the
electrical resistance of the electrical path between
auxiliary electrode 20 and measurement electrode 12 (or
test electrode 14) through electrolyte 16, thereby
requiring the output voltage produced by current driver
'-'O 94/0823, ~ ~ ~ ~ ~ ~ ~ PCT/US93/08836
- 21 -
74 to increase to maintain each electrode 12, 14 when
energized at the desired potential of -0.7 volts with
respect to reference electrode 22. Of course, driver 74
cannot produce an output voltage that exceeds its supply
voltage (e.g., +10 volts). When this maximum,level is~
,_
reached, driver 74 will be unable to compensate for
further increases in the resistance of electrolyte 16.
If this occurs, the potential on energized electrodes 12,
14 will fall, leading to errors in the oxygen measurement
l0 or fouling diagnosis. ' '
Comparator 192 monitors the output voltage 194 of
driver~74. If output voltage 194 exceeds a predetermined
percentage (e. g., 90%) of the supply voltage (in this
example, +9 volts), comparator 192 notifies processor 32.
Processor 32 responds to this error by generating a fault
196, which is displayed as an error message on display
34.
Still other embodiments are within the scope of
the following claims.
